1. Field of the Invention
This invention relates to a molten carbonate fuel cell, and more specifically to the porous anode thereof mainly consisting of a nickel-base alloy.
2. Description of Prior Art
A molten carbonate fuel cell is a DC electric power generating system comprising a unit cell consisting of an anode and cathode with an electrolyte plate therebetween. A plurality of unit cells are stacked with separator or bipolar plates therebetween to form a module to be operated under high pressure and at about 650.degree. C. The electrolyte plate is a mixture of electrolyte and electrolyte holding agent, the electrolyte being a eutectic mixture of carbonates of alkaline metals such as lithium or potassium, and the electrolyte holding agent, also called a matrix, comprising .gamma.-lithium aluminate or LiAlO.sub.2 to hold the eutectic mixture in a stable form.
The anode is a porous fuel electrode formed by sintering nickel or a nickel-base alloy. The cathode is a porous oxidant electrode consisting of nickel oxide doped with lithium. Both electrodes are gas diffusion electrodes. With a larger effective area of the three phase zones where the solid phase of electrode, the liquid phase of electrolyte, and the gas phase of reaction gas intersect, the overvoltage of electrochemical reaction for electric power generation becomes smaller, which results in superior performance of the cell.
For stable electric power-generation, the porosity and the size of the pore initially set up must be maintained for a long operating time.
In an anode operating under high pressure and high temperature, however, the porosity and pore distribution changes gradually due to compression creep of the electrode metal, causing the electrode thickness to decrease and the electrode characteristic to deteriorate. The decrease of the electrode thickness causes the contact resistance in the module to increase and promotes the voltage lowering. Known porous anodes for a molten carbonate fuel cell are made of nickel powder at the beginning, powders of nickel-base alloy such as Ni-Al or Ni-Cr, or a mixture of these alloy powders and ceramics powders, as is disclosed in the specification of U.S. Pat. No. 4,247,604.
Although the Ni-Al or Ni-Cr alloys have superior anti-creeping characteristic than nickel alone, the content of aluminum or chromium must be increased to obtain favorable characteristics. However, the increase of the content of aluminum or chromium causes the decrease of electric conduction of the alloys, thereby lowering cell voltage due to IR drop in the anode.
To suppress the over sintering of an anode in operation under high temperature and high pressure, on the other hand, addition of ceramic powder to these nickel-base alloys has been tried. However, the simple mixing of ceramic powder or the coating of alloy powder with ceramics was not enough to suppress the compression creeping of nickel-base alloy of the bulk metal forming the anode.